According to Jacobs, other research has shown that firms with unionized employees have diminished differences in pay – such that the gap in the earnings of the highest-paid worker and the lowest-paid workers was reduced in firms organized by unions.

“Unions were also the most effective political advocates forthe less affluent before Congress, the president and other elected officials,”Jacobs said. “They ended up helping less prosperous families even if they weren’t union members.”

The researchers appeared to have controlled for most factors that could possibly contribute to income inequality--some of which were, of course, factors. But they find that unionism was still quite significant. Probably not too surprising for many readers (heck, even with low union density rates, there's still a significant union wage premium), but it's useful to have that message spread more broadly.